Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Both require independent ink supplies for each of the colors of ink provided. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.
The first technology, commonly referred to as “droplet on demand” ink jet printing, selectively provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of a flying ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.
Conventional droplet on demand ink jet printers utilize a heat actuator or a piezoelectric actuator to produce the ink jet droplet at orifices of a printhead. With heat actuators, a heater, placed at a convenient location, heats the ink to cause a localized quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, a mechanical force causes an ink droplet to be expelled.
The second technology, commonly referred to as “continuous stream” or simply “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Traditionally, the ink droplets are selectively electrically charged. Deflection electrodes direct those droplets that have been charged along a flight path different from the flight path of the droplets that have not been charged. Either the deflected or the non-deflected droplets can be used to print on receiver media while the other droplets go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001, issued to Hansell, on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium.
In another form of continuous ink jet printing, such as is described in U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002, commonly assigned, included herein by reference, stimulation devices are associated with various nozzles of the printhead. These stimulation devices perturb the liquid streams emanating from the associated nozzle or nozzles in response to drop formation waveforms supplied to the stimulation devices by control means. The perturbations initiate the separation of a drop from the liquid stream. Different waveforms can be employed to create drops of a plurality of drop volumes. A controlled sequence of waveforms supplied to the stimulation device yields a sequence of drops, whose drop volumes are controlled by the waveforms used. A drop deflection means applies a force to the drops to cause the drop trajectories to separate based on the size of the drops. Some of the drop trajectories are allowed to strike the print media while others are intercepted by a catcher or gutter.
Having understood some basics of a continuous inkjet printer, a brief description of synchronizing ejected print droplets to the print media is useful. In this regard, one or more printheads are positioned adjacent to a print media such that the printhead is able to deposit ink or other printing fluid on the print media as the print media is moved relative to the printhead. In many such printing systems, the relative velocity of the print media past the printhead (print speed) can vary widely, for example from 50 ft/min. to 1000 ft/min. The velocities are given by way of example and are not limiting to the claimed invention. While the print speed can vary widely, continuous inkjet printers typically have a base drop creation rate or frequency that is fixed, or at least can not be varied widely. In some cases the base drop creation frequency is defined by a printing system clock or by a natural characteristic of the drop generator such as its resonant frequency. As drops can be printed only when drops are created, the time between successive drops that are printed is limited to values that are an integer number of the base drop creation periods. When the print speed is low, the time between successive printed drops corresponds to the base drop creation period times a large integer, while for high print speeds the time between successive print drops corresponds to the base drop creation period times a small integer.
In many types of continuous inkjet, a print drop can not be created at the base drop creation rate. For example in some printing systems that electro-statically deflect the non-print drops so that they strike the catcher, successive print drops must be separated by two or more catch drops. Similarly, by way of example, some print systems that separate print and catch drops by a means of a flow of gas across the drop trajectory, the print drops are formed from the ink that passes through the nozzle during not just one base drop creation period but rather in a plurality, typically three, of the base drop creation periods.
As a result, there are certain print speeds at which the pixel locations on the print media move past the printhead at a rate, called the pixel rate, which exactly matches a frequency at which printable drops can be created. At such speeds the print drop creation rate becomes synchronized with the pixel rate. At these speeds, the time between successive pixel locations on the pint media passing the printhead is equal to an integer N times the base drop creation period; where N must be 2 or 3 or more, depending on the drop deflection mechanism. For example, if the base-drop creation frequency is 360 kHz, N=3, and the print resolution is 600 drops per inch, this occurs at 200 in/sec print speed. FIG. 4A illustrates this. There are three fundamental drops 100 created at the base-drop creation frequency for each pixel spacing 102 so the drop formation is synchronized with the pixel rate. FIG. 4B illustrates a sequence of drops 104 printed in a print media in one such printer in which the print drops 104 have three times the volume of the non-print drops 100. Since the print drop formation is synchronized with the rate at which pixel moves past the printhead, the print drops are evenly spaced on the print media, landing at a consistent location within the respective pixels locations.
In addition to the 200 in/sec speed at which the pixel rate equaled the base drop creation frequency divided by N=3, other print speeds at which the pixel rate equals the base drop creation rate divided by other larger integer values allow the pixel rate to be synchronized with the print drop creation rate. For the same base drop frequency and print resolution as in the example above and using N=4, a print speed of 150 in/sec is required to match the pixel rate exactly with the print drop creation rate. FIG. 5A illustrates such a case, four fundamental drops 100 are created for each pixel spacing 102. The base drop creation rate is again synchronized with the pixel rate. FIG. 5B illustrates a sequence of print and catch drops where one print drop 104 is created for every four of the base drops 100 and where the print drop has a volume equal to three times the volume of the non-print base drops. A repeated pattern of one print drop 104 and one catch drop 100 are produced for each pixel location 102. Again the print drops are uniformly spaced and land at a consistent place within each pixel interval.
The printing system, however, needs to be able to print not just at those print speeds at which the pixel rate equals the print drop creation rate, but also at all intermediate speeds. For example, it must be able to print not just at 150 in/sec (where N=4) and 200 in/sec (where N=3), but also at print speeds between these two values. At such intermediate print speeds, the time between successive print drops is not fixed. The time between successive print drops is three times the base drop creation period for some of the drops, while other print drop pairs are separated by four times the base drop creation rate. FIG. 6A illustrates a sequence of the base drops 100 created at such a speed. During pixel intervals 1, 3, 4, 6, 7, 9, and 10, three drops 100 were created, while in pixel intervals 2, 5, and 8 four drops were created. When creating print drops to print in each of the pixels, it is necessary to account for this variation in number of base drops that could be created in the pixel time interval. Therefore as shown in FIG. 6B, during pixel intervals 1, 3, 4, 6, 7, 9, and 10, a single print drop 104 was created, while in pixel intervals 2, 5, and 8 one print drop 106 and one catch drop 100 were created. While this ensures that the print drops land within the proper pixel locations, the spacing between print drops is not consistent. This is seen more clearly in FIG. 6C, where the catch drops and the pixel interval markings have been removed to more clearly show how the print drops 106 would look on the print media. Some of the drops are separated from the preceding drop more than the other drops are from the drop that precedes them. These drop spacing intervals where the spacing is different (typically larger) than most of the drop spacing intervals are called synchronization bands or synch incidents 108.
As the print drops are not created at consistent time intervals, their spacing as they drop through the air is also not consistent. As a result the print drops do not encounter the same amount of air drag as they drop from the drop generator to the print media. The print drops 105 preceded by a synch incident 108 encounter more air drag than the other drops. The impact of these drops gets shifted as a result of the increased air drag, to produce a larger apparent synch band as seen in FIG. 6D. Depending on the print conditions, the synch bands may be readily observed by to a person looking at the print. A means to overcome the visibility of synch bands is desired. The present invention addresses this needed improvement.